1. Field of the Invention
The present invention relates to apparatus and methods for extracting representative samples of earth formation fluids. More particularly, the present invention relates to a tool for obtaining a sample of formation fluid and maintaining the sample in a single phase state until delivered to a testing laboratory.
2. Description of the Related Art
The physical properties of earth formation fluids vary greatly respective to geologically diverse formations. Properties such as chemical composition, viscosity, gaseous phase envelope and solid phase envelope greatly affect the value of a formation reservoir. Further, these properties affect decisions as to whether production may be economically achieved at all and, if so, the duration, expense and unit price of such production. For these reasons, paramount importance is assigned to the accuracy of reservoir fluid samples. Preservation of the in situ phase state of a sample is first among several accuracy criteria.
Various methods exist for extraction of a well sample. Among such methods are those that obtain separate samples of well fluids, liquid and gas, as produced at the well surface. These samples are combined in a manner believed to represent the in situ fluid. Petroleum reservoirs are usually several thousands of feet from the earth's surface and are typically under pressures of several thousands of pounds per square inch. Geothermal temperatures at these depths are on the order of 250° F. or more.
Due to such downhole environmental extremes, transfer of a formation fluid sample to the surface environment carries a possibility of inducing several irreversible changes in the sample. During the rise of a downhole fluid sample to the surface, both pressure and temperature drop dramatically. Such changes may cause certain components of the formation fluid to irreversibly precipitate from solution and/or colloidal suspension and thereby be underestimated by surface sampling. Well production events such as paraffin or asphaltene deposition, may cause substantial downhole damage to the well. Such damage might be entirely avoidable if accurate testing could determine the precise composition, pressure and temperature of the formation fluid. It is especially important for asphaltene studies, where the precipitation and subsequent removal of asphaltene is not well understood, that a formation fluid sample is kept above the saturation pressure to assure that the original composition is maintained.
However, prevention of irreversible changes in a formation sample during retrieval to the surface and discharge into pressurized test or storage devices has remained problematic. Early sample tools employed a fixed volume sample chamber that was initially evacuated. The evacuated sample chamber was lowered to the desired formation depth where a valve was remotely opened to allow an inflow of well fluid into the sample collection chamber. Once filled, the valve was closed for retention of the sample and the chamber retrieved to the surface. During retrieval of the sample tool to the surface, cooling of the sample, in a fixed volume, resulted in a sample pressure decrease. Decreased pressure often resulted in the gasification of certain fractional components of the sample as well as irreversible precipitation of certain solid components. While very careful laboratory studies could be conducted on at least a partially recombined sample and further testing could be performed on components irreversibly separated from the original sample, there persisted a margin of possible inaccuracy which was sometimes critical to very valuable production properties. As those skilled in the art know, some production properties of a formation fluid can be problematic and expensive for cleaning or reworking of the well. It may be difficult, if not impossible, to restore the well to production following a rework.
Efforts to limit or prevent phase changes in formation fluid samples during retrieval and transport to a laboratory or to pressurized storage devices have resulted in variable volume sample chamber tools of two broad groups:
A. Tools having a sample chamber made variable in volume by inclusion of an internal reservoir of compressible fluid therein; and,
B. Tools having a sampling chamber made variable in volume by means of a pressurized incompressible fluid. An elastic means such as gas or a spring is typically used to pressure the incompressible fluid, either directly or indirectly through an intermediate piston.
U.S. Pat. No. 3,859,850 issued Jan. 14, 1975 to Whitten, GB20127229 A published in 1979 by Bimond et al, U.S. Pat. No. 4,766,955 issued Aug. 30, 1988 to Petermann and U.S. Pat. No. 5,009,100 issued Apr. 23, 1991 to Gruber et al all disclose subsurface sampling tools that employ a sample chamber of the nature of tools described in group A above. Characteristic of these Group A tools is in the sample chamber. The volume of the sample chamber is, essentially, made elastic by means of a piston that is a moving reservoir wall for a trapped volume of compressed gas. The gas is further compressed internally when pressure outside of the reservoir is greater than internal pressure of the trapped gas reservoir. As the sample tool is lowered downhole, the reservoir of trapped gas, if lower in pressure than the downhole pressure, decreases in volume. Resultantly, a piston in the reservoir is displaced against the trapped gas volume. In theory, upon cooling and contraction of the sample (as by retrieval to the surface), the gas in the reservoir will reexpand and maintain pressure of the sample. However, in order for the volume of the reservoir of the trapped gas to reduce as the reservoir descends and therefore be capable of reexpanding on retrieval, its initial pressure must be somewhat less that bottom-hole pressure of the sample. Additionally, as the sample cools on retrieval, so does the trapped gas thereby further reducing the ability of the trapped gas to reexpand fully from downhole conditions. Thus, while tools of group A may be of some utility, at least for the purpose of limiting the amount of pressure losses in a fluid upon retrieval from downhole, they are inherently incapable of maintaining the sample at or above downhole pressure condition during retrieval. Such tools also fail to disclose leakproof piston seal design. The possibility of gas leakage is mentioned in Bimond et al. In order to detect and account for such leakage, Bimond et al teaches the use of a tracer gas, such as carbon tetra fluoride which is not found in the well sample.
As alternatives to the tools of group A are the tools of group B such as disclosed by GB2022554A by McConnachie, U.S. Pat. No. 5,337,822 issued Aug. 16, 1994 to Massie et al and U.S. Pat. No. 5,662,166 issued Sep. 2, 1997 to Shammai. These tools represent an improvement to the tools of group A in the sense that both have the capability of retrieving a sample while maintaining a sample pressure at or above original downhole pressure. Despite at least the possibility of improved performance, both tools, however, utilize an incompressible fluid to drive, either directly or indirectly, against a trapped volume of sampled fluid. Said piston is powered by an elastic source such as a gas or mechanical spring.
GB2022554A to McConnachie discloses a subsurface flow-through sampling tool. As the sampler descends in the well, well fluid enters and exits the sample chamber through flow-through ports. Once at the desired depth, Well fluid is trapped in the tool by a sliding dual piston means. Valve means then releases a pressurized gas that drives a piston for displacement of mercury under pressure into the sample chamber. The resulting sequence of pressure transmission forces to maintain pressure on the sample is: pressurize gas→piston→mercury→well sample.
U.S. Pat. No. 5,337,822 to Massie et al employs a sample chamber that is divided by a moveable piston. The sample chamber piston is pressurized against the sample by an incompressible fluid such as mineral oil. The mineral oil is, in turn, pressurized by a moveable piston contained in a second chamber. The moveable piston of the second chamber is, in turn, driven by an elastic source such as a gas or mechanical spring in said second chamber. The resulting sequence of pressure transmission forces to maintain pressure on the sample is; elastic sources→second pistons→incompressible fluids→first pistons→oil sample. The Massie tool employs numerous parts and relies on a lengthy sequential operation of multiple valves with the attendant possibility of malfunction.
The sample chamber free piston of U.S. Pat. No. 5,662,166 to Shammai is loaded on the backside by a closed volume of hydraulic fluid. A remotely operated valve opens the closed hydraulic chamber for displacement into a secondary hydraulic chamber thereby permitting the downhole pressure against the front face of the sample chamber piston to displace the piston with a well fluid sample. At a predetermined piston displacement location, gas from a high pressure gas chamber is first released to close the hydraulic conduit between the sample chamber piston backside volume into the secondary hydraulic chamber and sequentially open the high pressure gas source into the piston backside volume to impose a standing compressive load on the sample.
Each of the aforesaid sample tool designs are either limited in performance or inherently complex, costly, likely to require substantial maintenance and are prone to malfunction. Accordingly, it is an object of the present invention to provide an improved tool for taking downhole samples of fluids in an earth borehole. Another object of the invention is to provide a downhole sampling tool capable of maintaining the in situ pressure of a sample at or above the downhole pressure during retrieval of the sample to the surface. Also an object of the invention is a sampling tool that minimizes heat loss from a sample during the well retrieval interval while maintaining high pressure on the sample to offset significant cooling upon retrieval. Another object of the invention is provision of a means for adding a gas accumulator to thermally stabilized sample tanks which are balanced to hydrostatic pressure. Stabilizing the temperature near formation temperature allows a gas accumulator and initial pressure settings to be designed to keep the sample pressure above or equal to formation pressure as the sample cools to the eutectic temperature. An additional object of the present invention is to provide a downhole sampling tool of simple, efficient, reliable and inexpensive design characteristics.